Magnetic Random Access Memory (MRAM) is a non-volatile data memory technology that stores data using magnetoresistive cells such as Magnetoresistive Tunnel Junction (MTJ) cells. At their most basic level, such MTJ elements include first and second magnetic layers that are separated by a thin, non-magnetic insulating layer such as a tunnel barrier layer, which can be constructed of a material such as MgO. The first magnetic layer, which can be referred to as a reference layer, has a magnetization that is fixed in a direction that is perpendicular to that plane of the layer. The second magnetic layer, which can be referred to as a magnetic free layer, has a magnetization that is free to move so that it can be oriented in either of two directions that are both generally perpendicular to the plane of the magnetic free layer. Therefore, the magnetization of the free layer can be either parallel with the magnetization of the reference layer or anti-parallel with the direction of the reference layer (i.e. opposite to the direction of the reference layer).
The electrical resistance through the MTJ element in a direction perpendicular to the planes of the layers changes with the relative orientations of the magnetizations of the magnetic reference layer and magnetic free layer. When the magnetization of the magnetic free layer is oriented in the same direction as the magnetization of the magnetic reference layer, the electrical resistance through the MTJ element is at its lowest level (low resistance state). Conversely, when the magnetization of the magnetic free layer is in a direction that is opposite to that of the magnetic reference layer, the electrical resistance across the MTJ element is at its highest level (high resistance state).
The switching of the MTJ element between high and low resistance states results from electron spin torque transfer. Generally, electrons flowing through a conductive material have random spin orientations with no net spin polarization. However, when electrons flow through a magnetized layer, the spin orientations of the electrons become aligned so that there is a net polarization of the current, and the orientation of this alignment is dependent on the orientation of the magnetization of the magnetic layer through which they travel. When the magnetizations of the free and reference layer are oriented in the same direction, the spin of the electrons in the free layer are in generally the same direction as the orientation of the spin of the electrons in the reference layer. Because these electron spins are in generally the same direction, the electrons can pass relatively easily through the tunnel barrier layer. However, if the orientations of the magnetizations of the free and reference layers are opposite to one another, the spin of the electrons in the free layer will be generally opposite to the spin of electrons in the reference layer. In this case, electrons cannot easily pass through the barrier layer, resulting in a higher electrical resistance through the MTJ stack.
Because the MTJ element can be switched between low and high electrical resistance states, it can be used as a memory element to store a bit of data. For example, the low resistance state can be read as an off or “0”, whereas the high resistance state can be read as a “1”. In addition, because the magnetic orientation of the free layer remains stable without any electrical power to the element, it provides a robust, non-volatile data memory bit.
To write a bit of data to the MTJ cell, the magnetic orientation of the magnetic free layer can be switched from a first direction to a second direction that is 180 degrees from the first direction. This can be accomplished, for example, by applying a current through the MTJ element in a direction that is perpendicular to the planes of the layers of the MTJ element. An electrical current applied in one direction will switch the magnetization of the free layer to a first orientation, whereas an electrical current applied in a second direction will switch the magnetic of the free layer to a second, opposite orientation. Once the magnetization of the free layer has been switched by the current, the state of the MTJ element can be read by reading a voltage across the MTJ element, thereby determining whether the MTJ element is in a “1” or “0” bit state. Advantageously, once the switching electrical current has been removed, the magnetic state of the free layer will remain in the switched orientation until such time as another electrical current is applied to again switch the MTJ element. Therefore, the recorded data bit is non-volatile in that it remains intact in the absence of any electrical power. The amount of current needed to switch the MTJ element is significantly larger than the amount needed to read it. During the write operation, the current flowing through the pillar causes significant self-heating in the proximity of the tunneling barrier.